linux-kernel - Re: [PATCH v2 4/8] KVM: VMX: Optimize vmx_set

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Date:   Wed, 9 Oct 2019 12:40:53 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Reto Buerki <reet@...elabs.ch>,
        Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH v2 4/8] KVM: VMX: Optimize vmx_set_rflags() for
 unrestricted guest

On 27/09/19 23:45, Sean Christopherson wrote:
> Rework vmx_set_rflags() to avoid the extra code need to handle emulation
> of real mode and invalid state when unrestricted guest is disabled.  The
> primary reason for doing so is to avoid the call to vmx_get_rflags(),
> which will incur a VMREAD when RFLAGS is not already available.  When
> running nested VMs, the majority of calls to vmx_set_rflags() will occur
> without an associated vmx_get_rflags(), i.e. when stuffing GUEST_RFLAGS
> during transitions between vmcs01 and vmcs02.
> 
> Note, vmx_get_rflags() guarantees RFLAGS is marked available.

Slightly nicer this way:

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 8de9853d7ab6..62ab19d65efd 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1431,9 +1431,17 @@ unsigned long vmx_get_rflags(struct kvm_vcpu *vcpu)
 void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
-	unsigned long old_rflags = vmx_get_rflags(vcpu);
+	unsigned long old_rflags;
+
+	if (enable_unrestricted_guest) {
+		__set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail);
+		vmx->rflags = rflags;
+		vmcs_writel(GUEST_RFLAGS, rflags);
+		return;
+	}
+
+	old_rflags = vmx_get_rflags(vcpu);
 
-	__set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail);
 	vmx->rflags = rflags;
 	if (vmx->rmode.vm86_active) {
 		vmx->rmode.save_rflags = rflags;

Paolo